Investigations into the effects of biofilm formation on the transport behavior of microplastics in open channel flows

The presence of biofilm on microplastics (MPs) influences their mobility in aquatic environments by modifying critical physical attributes such as size and density. This study examines how biofilm-induced alterations affect the transport characteristics of polyethylene (PE) and polyester in open-channel flow. A laboratory incubation experiment lasting 7 weeks was performed under a regulated light-dark cycle to observe biofilm growth on low-density polyethylene (LDPE). Findings revealed that biofilm thickness increased rapidly within the first 7 days and stabilized by day 21. Subsequently, an empirical growth equation was established from these observations and consistently applied to both PE and polyester particles. Employing this calibrated model, particle tracking simulations were implemented for MPs (25 to 200 μm) within fully developed open-channel flows. The simulation results demonstrate that biofilm accumulation markedly enhances the settling propensity of both polymers. Biofouled PE particles with an initial diameter of 100 μm showed increased vertical movement compared to smaller polyester particles, and PE particles below 50 μm also exhibited more pronounced settling than uncoated polyester despite having lower density. These findings reveal that biofilm development can intensify the sinking of inherently buoyant MPs, thereby altering their transport processes. The results underscore the importance of accounting for biofilm-mediated modifications in both particle size and density to enhance the accuracy of MP fate predictions in riverine systems.